Arrangement and method for controlling combustion in a combustion engine

ABSTRACT

An arrangement and a method for controlling a combustion engine, e.g. of the type called HCCI engine. A control unit is operable for controlling the self-ignition of the fuel mixture towards an optimum crankshaft angle (cad iopt ) within a load range (L tot ). The load range (L tot ) can be divided into at least two subranges (L II , L III ). The control unit is operable to controlling the self-ignition of the fuel mixture towards an optimum crankshaft angle (cad iopt ) within one of the subranges (L II ) by a strategy (II) which entails the effective compression ratio (c) in the cylinder being varied, and within the second subrange (L III ) by another strategy (III) which entails a variable amount of cooled exhaust gases (ceg) being led to the combustion chamber also enabling in the second subrange (L III ) to control the self-ignition of the fuel mixture towards an optimum crankshaft angle (cad iopt ) by variation of the effective compression ratio (c) in the cylinder without it falling below a lowest acceptable value (C min ).

CROSS REFERENCE TO RELATED APPLICATION

The present application is a 35 U.S.C. §§ 371 national phase conversionof PCT/SE2004/001212, filed 19 Aug. 2004, which claims priority ofSwedish Application No. 0302247-2, filed 20 Aug. 2003. The PCTInternational Application was published in the English language.

BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The present invention relates to an arrangement and a method forcontrolling a combustion engine and particularly relates to strategiesfor adjusting the combustion process responsive to certain engineconditions.

One type of such combustion engines are called HCCI (Homogeneous ChargeCompression Ignition) engines and may be regarded as a combination of anOtto engine and a diesel engine. In HCCI engines, a homogeneous mixtureof fuel and air is compressed in a combustion chamber untilself-ignition of the fuel mixture takes place. Advantages of suchengines are that they produce low discharges of nitrogen oxides NO_(x)and soot particles while at the same time being of high efficiency. Onereason for HCCI engines not being used conventionally to any greatextent is that it is difficult to control the self-ignition of the fuelmixture to a correct crankshaft angle.

Two different valve control strategies are known for controlling underlaboratory conditions the self-ignition of the fuel mixture. The firststrategy refers to closing the exhaust valve before the combustionchamber has been emptied of exhaust gases from a preceding combustionprocess and to opening the inlet valve later than usual. Such aso-called negative overlap results in a varying amount of exhaust gasesbeing retained in the combustion chamber for a subsequent combustionprocess. The hot exhaust gases retained in the combustion chamber raisethe temperature of the fuel mixture for the subsequent combustionprocess. A suitable amount of exhaust gases retained can thus cause thefuel mixture to have an initial temperature such that it self-ignites ata substantially optimum crankshaft angle.

The second strategy refers to controlling the closing of the inletvalve. The compression ratio in the cylinder can be varied by varyingthe crankshaft angle at which the inlet valve closes. The later theinlet valve closes, the shorter the piston movement required forcompression of the fuel mixture. Varying the inlet valve closure andhence the effective compression ratio in the cylinder makes it possiblefor self-ignition of the fuel mixture to take place at a substantiallyoptimum crankshaft angle.

A disadvantage of the aforementioned strategies is that they providecontrol of self-ignition in an HCCI engine within a relatively limitedload range. Most technical applications need an engine which can be usedover a relatively large load range.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an arrangement and amethod for providing effective control of the self-ignition of acombustion engine of the kind mentioned in the introduction so that itcan be used across a relatively large load range.

This object is achieved with our arrangement varies the combustionprocess responsive to certain engine conditions. This involves applying,within a subrange, a strategy which entails the effective compressionratio in the cylinder being varied. By suitable variation of thecompression ratio in the cylinder, self-ignition of fuel mixtures fordifferent loads can be caused to take place at a substantially optimumcrankshaft angle. This strategy may be applied in a subrange withinwhich there are a lowest load at which an optimum compression ratioprevails and a highest load at which the effective compression ratio hasbeen reduced to a minimum acceptable value. Such limitation to a minimumacceptable value is necessary in cases where reducing the effectivecompression ratio causes the lambda value to drop and hence the acidcontent A3

This object is achieved with our arrangement varies the combustionprocess responsive to certain engine conditions. This involves applying,within a subrange, a strategy which entails the effective compressionratio in the cylinder being varied. By suitable variation of thecompression ratio in the cylinder, self-ignition of fuel mixtures fordifferent loads can be caused to take place at a substantially optimumcrankshaft angle. This strategy may be applied in a subrange withinwhich there are a lowest load at which an optimum compression ratioprevails and a highest load at which the effective compression ratio hasbeen reduced to a minimum acceptable value. Such limitation to a minimumacceptable value is necessary in cases where reducing the effectivecompression ratio causes the lambda value to drop and hence the acidcontent of the exhaust gases to decrease. Lowering the lambda valueresults in corresponding pressure rises and increased emissions. Athigher loads than this strategy caters for, the control unit applies astrategy which entails cooled exhaust gases being led to the combustionchamber. The cooled exhaust gases cause the ignition of the fuel mixtureto take place later. This means that the control unit can raise theeffective compression ratio in the cylinder and the lambda value,resulting in the possibility of more fuel being supplied to the fuelmixture in the combustion chamber, and in a higher engine load beingachieved. This strategy is therefore applicable within a load rangewhich is higher than and adjacent to the load range for the strategywhich only entails the effective compression ratio in the cylinder beingvaried. The control unit applying different strategies within variousmutually adjacent subranges makes it possible for self-ignition to becontrolled towards an optimum crankshaft angle within a relatively largeload range.

According to a preferred embodiment of the present invention, thecontrol unit is adapted to regulating the effective compression ratio inthe cylinder by initiating variable inlet valve closure. Inlet valveclosure variation is an uncomplicated way of regulating the effectivecompression ratio. The later the inlet valve closes, the shorter thepiston movement in the cylinder required for compressing the fuelmixture. With advantage, the arrangement comprises at least onehydraulic control system for lifting the inlet valve and the exhaustvalve. Such hydraulic systems which quickly vary the inlet valve closurefrom one combustion process to another in response to control signalsreceived from the control unit are conventionally available.

According to a preferred embodiment of the present invention, thearrangement comprises a return line extending from an exhaust line ofthe combustion engine to an inlet line for air supply to the combustionchamber. This enables exhaust gases from previous combustion processesto be mixed in with the air and led to the combustion chamber. Withadvantage, the return line comprises a valve for controlling the supplyof exhaust gases to the inlet line. In such cases the control unitcontrols the valve so that a specified amount of exhaust gases issupplied to the combustion chamber. The return line preferably comprisesa cooler for cooling the exhaust gases before they reach the inlet line.Such a cooler enables the exhaust gases to be brought to a substantiallyspecified temperature before they are led into the combustion chamber.

According to another preferred embodiment of the present invention, thearrangement comprises a first sensor for detecting a parameter whichindicates the start of a combustion process in the combustion chamber,and a second sensor for estimating the crankshaft angle of thecombustion engine, and the control unit is adapted to determining thecrankshaft angle at the start of the combustion process. Said firstsensor may be a pressure sensor which detects the pressure in thecombustion chamber. The control unit can use information about thepressure characteristic in the combustion chamber for determining thecrankshaft angle at which the self-ignition has taken place. The firstsensor may alternatively be a sonic sensor or some other suitable sensorby which self-ignition in the combustion chamber can be detected. Thesecond sensor may be a sensor which detects the rotational position of,for example, the engine's flywheel. The control unit is preferablyadapted to comparing the actual crankshaft angle at the self-ignition ofthe combustion process with stored information concerning the optimumcrankshaft angle for self-ignition of the combustion process and tousing this information for controlling the self-ignition of thefollowing combustion process. The control unit can calculate thedifference between the actual crankshaft angle at self-ignition of thecombustion process with stored information about the optimum crankshaftangle. Thereafter the control unit controls the lifting of the valves insuch a way as to eliminate during the next combustion process anydifference thus calculated.

According to another preferred embodiment of the present invention, thearrangement comprises an inlet line for air supply to the combustionchamber, and an injection nozzle for fuel injection into the combustionchamber. In this case, the air and the fuel are supplied separately to,and become mixed in, the combustion chamber. Alternatively, the air andfuel may be mixed outside to form fuel mixture and be led together intothe combustion chamber.

The object of the invention is also achieved by the method of the kindmentioned in the introduction which is characterised by the featuresindicated in the characterising part of claim 11. Using two differentstrategies for controlling the self-ignition of the fuel mixture withindifferent but mutually adjacent load subranges enables continuouscontrol, within a relatively broad load range, of the self-ignition ofthe type of combustion engines usually called HCCI engines.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below by way ofexample with reference to the attached drawings, in which:

FIG. 1 depicts a combustion engine with an arrangement according to thepresent invention,

FIG. 2 depicts valve lifting of a combustion engine according to a firststrategy,

FIG. 3 depicts valve lifting of a combustion engine according to asecond strategy,

FIG. 4 depicts the effective compression ratio as a function of thecrankshaft angle at inlet valve closure,

FIG. 5 depicts schematically three load subranges of a combustion enginecontrolled by three different strategies and

FIG. 6 depicts a flowchart describing a method for controlling theself-ignition of a combustion engine.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 depicts an arrangement for controlling a combustion engine 1 ofthe type in which a homogeneous mixture of fuel and air is compresseduntil self-ignition of the mixture takes place due to the heat arisingduring the compression. Such an engine 1 is usually called an HCCI(Homogeneous Charge Compression Ignition) engine. An HCCI engine may beregarded as a combination of an Otto engine and a diesel engine. Acylinder 2 of the engine 1 is depicted here. The engine 1 may of coursehave substantially any desired number of such cylinders 2. The engine 1comprises a combustion chamber 3 which is bounded downwards in thecylinder 2 by a movable piston 4. The piston 4 is connected to acrankshaft 5 by a connecting rod 6. The movements of the piston 4 in thecylinder 2 are converted to rotary movement of the crankshaft 5.

When the piston 4 moves downwards in the cylinder 2 and an inlet valve 8is open, air is drawn into the expanding combustion chamber 3 via aninlet line 7. At the same time, a fuel pump 9 injects fuel into thecombustion chamber 3 via an injection nozzle 10. The inlet valve 8usually closes at the stage when the piston 4 changes direction at anextreme bottom position. The subsequent upward movement of the piston 4causes compression of the fuel mixture in the combustion chamber 2. Thefuel mixture undergoes a temperature increase which is related to thedegree of compression. Substantially at the stage when the piston 4passes an extreme top position in the cylinder 2, the fuel mixtureshould have reached the temperature at which self-ignition of the fuelmixture takes place. During the combustion process, powerful expansionoccurs in the combustion chamber 3 and the piston 4 is pushed downwards.When the piston 4 has passed the extreme bottom position, an exhaustvalve 11 opens. The piston 4 moving upwards then pushes the exhaustgases formed during the combustion process out via the exhaust valve 11to an exhaust line 12.

The arrangement comprises a return line 13 for recirculation of exhaustgases which extends from the exhaust line 12 to the inlet line 7. Thereturn line 13 comprises a valve 14 and a cooler 15. The arrangementalso comprises a pressure sensor 16 adapted to detecting the pressure inthe combustion chamber 3 and a sensor 17 adapted to detecting therotational position of the crankshaft 5. The sensor 17 may for exampledetect the position of the engine's flywheel. The arrangement comprisesa schematically depicted hydraulic system 18 a for lifting the inletvalve 8 and a schematically depicted hydraulic system 18 b for liftingthe exhaust valve 11. In this case, the lifting of the inlet valve 8 andthe exhaust valve 11 takes place independently of the rotationalposition of the crankshaft. The arrangement comprises a control unit 19adapted to controlling the engine 1 so that self-ignition of the fuelmixture takes place at an optimum crankshaft angle. The control unit 19is adapted to receiving signals from the sensors 16, 17 and to sendingcontrol signals to the hydraulic systems 18 a, b so that the lifting ofthe inlet valve 8 and the exhaust valve 11 takes place at desirablecrankshaft angles. The control unit 19 may be a computer unit withsuitable software.

FIG. 2 has continuous lines depicting a lifting phase d of an inletvalve 8 and a lifting phase d of an exhaust valve 11 as a function ofthe crankshaft angle cad (crank angle degree) when there is conventionalcontrol of the inlet valve 8 and the exhaust valve 11. In this case, theinlet valve opening ivo takes place substantially at the extreme topposition of the piston 4 at a crankshaft angle here designated as 0°.The inlet valve closure ivc takes place just after the piston has passedthe extreme bottom position at a crankshaft angle of 180°. In this case,the exhaust valve opening evo takes place at a crankshaft angle of about500° and the exhaust valve closure evc takes place substantially at thepiston's extreme top position at a crankshaft angle of 720°. As theengine 1 is a four-stroke engine, its working cycle comprises crankshaftrotation through 720°. Crankshaft angles of 0° and 720° are thusequivalent from the working cycle point of view. With conventional valvecontrol, the exhaust valve closure evc and the inlet valve opening ivotake place substantially simultaneously or with a slight overlap so thatthe combustion chamber is emptied of exhaust gases after a combustionprocess. The optimum crankshaft angle cad_(iopt) for self-ignition ofthe fuel mixture is substantially immediately after the piston 4 haspassed the extreme top position at a crankshaft angle of 360°. Thedifficulty of supplying a fuel mixture which will self-ignitesubstantially exactly at the optimum crankshaft angle cad_(iopt) is acontributory cause of HCCI engines having substantially not yet comeinto conventional use.

A first strategy I known per se for controlling the self-ignition of thefuel mixture to the optimum crankshaft angle cad_(iopt) is to close theexhaust valve 11 before the piston 4 reaches the extreme top position at720° and to open the inlet valve 8 after the piston 4 has passed theextreme top position at 0°. Such valve lifting involving early exhaustvalve closure evc′ and late inlet valve opening ivo′ is represented bydiscontinuous lines in FIG. 2. Early exhaust valve closure evc′ and lateinlet valve opening ivo′ cause a so-called negative overlap during acrankshaft angle range within which both the inlet valve 8 and theexhaust valve 11 are closed. In this case, the exhaust valve 11 isclosed during a crankshaft angle range a before 720° and the inlet valveis closed during a crankshaft angle range b after 0°. The resultingnegative overlap will be the aggregate of the crankshaft angle ranges aand b. Early exhaust valve closure evc′ means that the combustionchamber 3 will not be entirely emptied of exhaust gases and that acertain amount of exhaust gases will be retained in the combustionchamber 3. Late opening of the inlet valve 8 means that the residualpressure of the exhaust gases will be reduced to a level such that theydo not flow out through the inlet valve 8 when it opens. The negativeoverlap thus results in hot exhaust gases from a combustion processbeing retained in the combustion chamber until a subsequent combustionprocess. The hot exhaust gases retained heat the fuel mixture, causingearlier self-ignition. Suitable control of the inlet valve 8 and theexhaust valve 11 can be applied to cause a variable amount of exhaustgases to be retained in the combustion chamber 3 so that theself-ignition of the subsequent combustion process takes placesubstantially at the optimum crankshaft angle cad_(iopt).

A second strategy II known per se for controlling the self-ignition offuel mixtures at different loads to a substantially optimum crankshaftangle cad_(iopt) is to provide late inlet valve closure ivc′. FIG. 3 hascontinuous lines depicting a lifting phase d of the inlet valve 8 and alifting phase d of exhaust valve 11 as a function of the crankshaft'sangle of rotation cad (crank angle degree) when there is conventionallifting of the inlet valve 8 and the exhaust valve 11. Valve liftingresulting in late inlet valve closure ivc′ is represented bydiscontinuous lines. In other respects the valve lifting according tostrategy II takes place in a conventional manner as represented by thecontinuous line. Closing the inlet valve 8 at a late crankshaft angleivc′ reduces the piston movement required to compress the fuel mixtureand results in a reduced effective compression ratio in the cylinder 2.

FIG. 4 shows how the effective compression ratio c varies as a functionof inlet valve closure ivc at different crankshaft angles cad. It showsan optimum effective compression ratio c resulting from inlet valveclosure ivc_(opt) just after a crankshaft angle of 180°. Earlier orlater than optimum inlet valve closure ivc results in a lower effectivecompression ratio c. A lower effective compression ratio c means thatthe compression to which the fuel mixture is subjected in the cylinder 2is reduced, but the effective compression ratio c should not go below aminimum value c_(min). Reduced effective compression ratio means thatthe lambda value, which can be measured by a lambda probe on the engine1, falls, i.e. the sulphur content of the exhaust gases decreases.Lowering the lambda value results in corresponding pressure rises andincreased emissions. The inlet valve closure ivc should therefore notdeviate too much from the optimum inlet valve closure ivc_(opt). FIG. 4shows a maximum inlet valve closure ivc_(max) not to be exceeded and aminimum inlet valve closure ivc_(min) not to be undershot, in order toavoid going below the lowest acceptable effective compression ratioc_(min). Thus later than optimum inlet valve closure ivc can be variedwithin a crankshaft angle range e and earlier than optimum inlet valveclosure ivc can be varied within a crankshaft angle range f. Reducedcompression ratio in the cylinder 2 results in delayed self-ignition.Controlling the inlet valve closure ivc to a crankshaft angle which issuitably far from the optimum ivc_(opt) results in a reduced compressionratio in the cylinder 2 so that self-ignition takes place at an optimumcrankshaft angle cad_(iopt).

When the load of the engine 1 corresponds to an exactly idealcombination of fuel and air, the self-ignition of the fuel mixture dueto compression heat takes place at the optimum crankshaft anglecad_(iopt). When the load of the engine 1 is lower than ideal and thefuel mixture is leaner than ideal, the fuel mixture will not self-igniteby compression heat. In this case, strategy I can be applied to supplyhot exhaust gases in a suitable quantity for raising the fuel mixturetemperature so that self-ignition takes place at the optimum crankshaftangle cad_(iopt). When the load of the engine 1 is higher than ideal andthe fuel mixture richer than ideal, self-ignition due to compressionheat takes place too early. In this case, strategy II can be applied forsuitable reduction of the effective compression ratio c in the cylinder2 so that self-ignition is delayed and takes place at the optimumcrankshaft angle cad_(iopt). Strategy I and strategy II are thusapplicable within separate but mutually adjacent load ranges. Applyingstrategy I to leaner than ideal fuel mixtures and strategy II to richerthan ideal fuel mixtures enables optimum self-ignition within arelatively large load range.

As the effective compression range c should not be limited too much,strategy II is not applicable for loads over a certain value. Thecomposition of fuel mixtures for such high loads will be such that theyself-ignite before the optimum crankshaft angle cad_(iopt) even whenthere is maximum acceptable reduction of the effective compression ratioc_(min). In this case a third strategy III may be applied. Strategy IIIentails cooled exhaust gases being led to the combustion chamber 3. Thecooled exhaust gases cause the fuel mixture to ignite later. The controlunit 19 can thus raise the effective compression ratio c by closing theinlet valve 8 somewhat closer to the optimum crankshaft angle ivc_(opt).The acid content of the exhaust gases will rise and hence also thelambda value. The control unit 19 can then supply more fuel to thecombustion chamber 3 to achieve a higher engine load. Supplying cooledexhaust gases causes the inlet valve closure ivc to shift to the leftalong the curve in FIG. 4 to an ivc value situated between ivc_(max) andivc_(opt). Strategy III thus makes it possible also within this highload range to control the self-ignition of the fuel mixture by variationof the effective compression ratio c in the cylinder 2 without goingbelow the lowest acceptable effective compression ratio c_(min).Strategy III is thus applicable within a load range which is higher thanthe load range for strategy II. Applying control which comprises bothstrategy II and strategy III enables the control unit to control theself-ignition of fuel mixtures towards an optimum crankshaft anglecad_(opt) within a relatively large load range.

With advantage, all three strategies I, II, II are applied to controlthe engine 1 within a load range L_(tot) comprising the three subrangesL_(I), L_(II), L_(III). FIG. 5 depicts the three subranges L_(I),L_(II), L_(III) schematically as a function of load L and engine speedrpm. Strategy I is applied in no-load and low-load running, strategy IIin medium-load running and strategy m in high-load running. The variousstrategies I, II and III may thus be applied within the subranges L_(I),L_(II), L_(III), which do of course overlap one another. Thus thecontrol unit 19 can provide continuous control of the self-ignition ofthe engine 1 across a broad load range L_(tot).

FIG. 6 depicts a flowchart describing a method for controlling theengine 1. At step 20 the engine starts up. At step 21 a combustionprocess takes place in the combustion chamber 3. The pressure sensor 16detects the pressure characteristic in the combustion chamber 3. Thepressure sensor 16 sends substantially continuously to the control unit19 signals concerning the prevailing pressure in the combustion chamber3. The control unit 19 also receives from the sensor 17 informationconcerning the current crankshaft angle. At step 22 the control unit 19uses information about the pressure p in the combustion chamber 3 andthe crankshaft angle cad to calculate the crankshaft angle cad_(i) atwhich the self-ignition of the combustion process has taken place. Thecontrol unit 19 comprises stored reference values concerning an optimumcrankshaft angle cad_(i,opt) at which self-ignition should take place.At step 23 the control unit 19 compares the actual crankshaft angle cad;at self-ignition and the optimum crankshaft angle cad_(i,opt) forself-ignition. If cad_(i) is greater than cad_(i,opt), the combustionprocess started late and the control unit 19 is adapted to taking actionfor promoting earlier self-ignition in the subsequent combustionprocess. If cad_(i) is smaller than cad_(i,opt), the combustion processstarted early and the control unit 19 is adapted to taking action forpromoting later self-ignition in the next combustion process.

At step 24 the control unit 19 estimates whether it is possible tocontrol by means of strategy I the self-ignition of the subsequentcombustion process. If cad_(i) is greater than cad_(i,opt), the latestcombustion process started late and a somewhat larger amount of hotexhaust gases should therefore have been supplied to the combustionprocess. If cad_(i) is smaller than cad_(i,opt) the latest combustionprocess started early and a somewhat smaller amount of hot exhaust gasesshould therefore have been supplied to the combustion process. At step25 the control unit 19 initiates new values for the exhaust valveclosure evc′ and the inlet valve opening ivo′ so that an adjusted amountof exhaust gases is retained in the combustion chamber during thesubsequent combustion process. At step 26 the control unit 19 initiatesinlet valve closure at the crankshaft angle ivc_(opt) at which therewill be optimum compression in the cylinder 2. If it is not possible toreduce further the amount of exhaust gases retained in the combustionchamber, it may be found that the load is too high for strategy I to beapplicable for controlling the self-ignition of the subsequentcombustion process to the optimum crankshaft angle for self-ignitioncad_(i,opt).

At step 27, if the self-ignition cannot be controlled by means ofstrategy I, there is estimation of whether it is possible to control theself-ignition by applying strategy II. Strategy II entails inlet valveclosure ivc′ earlier or later than the optimum ivo_(opt). This meansthat the effective compression ratio c in the cylinder 2 can be reducedand the self-ignition delayed. Strategy II may thus be applied when thecharacteristics of the fuel mixture supplied are such that itself-ignites at too early a crankshaft angle during the compression inthe cylinder 2. The effective compression ratio c should thus not belowered below a minimum value c_(min). The inlet value closure ivc′ istherefore limited to the crankshaft angle ranges e, f depicted in FIG.4. If the control unit 19 estimates an inlet valve closure ivc′ which isneither above ivc_(max) nor below ivc_(min), strategy II may be appliedfor controlling the self-ignition. If cads is greater than cad_(i,opt),the latest combustion process started late, so the control unit 19adjusts the inlet valve closure ivc′ of the subsequent combustionprocess to a suitable extent towards ivc_(opt) in order to raise thecompression ratio c in the cylinder 2. If, on the contrary, cad_(i) issmaller than cad_(i,opt), the latest combustion process started early,so the control unit adjusts the inlet valve closure ivc′ of thesubsequent combustion process to a suitable extent away from ivc_(opt)in order to reduce further the compression ratio c in the cylinder 2. Ifthe new ivc′ value calculated by the control unit 19 falls within thecrankshaft angle ranges e, f, it is therefore possible to apply strategyII for controlling the subsequent combustion process. In that case, atstep 28 the control unit 19 initiates closure of the inlet valve 8 atthe calculated inlet valve closure ivc′. At step 29 the control unit 19initiates exhaust valve closure evc_(opt) and inlet valve openingivo_(opt) at crankshaft angles which result in minimum fuel consumption.The exhaust valve opening evo is controlled to a suitable value byoverall engine parameters which are independent of strategy II.

If the control unit 19 estimates an ivc′ value which does not fallwithin the crankshaft angle ranges e, f, it is not appropriate simply touse a reduced compression ratio for controlling the self-ignitiontowards the optimum crankshaft angle cad_(iopt). In such cases thecomposition of the fuel mixture will be such that controlling theself-ignition towards the optimum crankshaft angle cad_(i,opt) wouldentail reducing the compression ratio c to a value below c_(min). Atstep 30 the control unit 19 therefore applies strategy III, whichentails cooled exhaust gases being led to the combustion chamber. Ifcad_(i), is greater than cad_(i,opt), the latest combustion processstarted late and the control unit controls the valve 14 so that asmaller amount of cooled exhaust gases is led to the subsequentcombustion process. If cad_(i), is smaller than cad_(i,opt), thecombustion process started early and the control unit controls the valve14 so that a larger amount of cooled exhaust gases is led to thesubsequent combustion process. At step 31 the control unit thencalculates the amount of cooled exhaust gases ceg which should besupplied to the combustion chamber 3 for self-ignition of the fuelmixture to take place at the optimum crankshaft angle cad_(i,opt).Supplying a suitable amount of cooled exhaust gases ceg results in latercombustion of the fuel mixture. The ivc value is thus shifted to theleft along the curve in FIG. 4 to an ivc value situated betweenivc_(max) and ivc_(opt). At step 32 the control unit 19 raises thecompression ratio c by initiating an inlet valve closure ivc′ which issituated between the optimum inlet valve closure ivc_(opt) andivc_(max). The lambda value is thus raised, enabling more fuel to besupplied to the combustion chamber and a higher engine load to beachieved. At step 33 the control unit 19 initiates exhaust valve closureevc_(opt) and inlet valve opening ivo_(opt) at crankshaft angles whichresult in minimum fuel consumption. The exhaust valve opening evo iscontrolled by overall engine parameters which are independent ofstrategy III.

The invention is in no way limited to the embodiment to which thedrawings refer but may be varied freely within the scopes of the claims.The combustion engine need not be an HCCI engine but may be any desiredcombustion engine in which a homogeneous fuel mixture self-ignites bycompression. The drawings refer to one cylinder of the combustion engine1 but the number of cylinders may of course be varied, as also thenumber of other components such as valves, injection means etc.

1. An arrangement for controlling a combustion engine wherein thecombustion engine comprises a combustion chamber, a movable piston inthe combustion chamber and the piston being movable to compress a fuelmixture in the combustion chamber so that self-ignition of the fuelmixture takes place, a crankshaft connected to and driven by movementsof the piston; an inlet valve to the combustion chamber and an exhaustvalve from the combustion chamber; a control unit operable forcontrolling the self-ignition of the fuel mixture to an optimumcrankshaft angle (cad_(iopt)) of the crankshaft within a load range(L_(tot)), wherein the load range (L_(tot)) is divided into at least twosubranges (L_(II), L_(III)) and the control unit is operable forcontrolling the self-ignition of the fuel mixture towards the optimumcrankshaft angle (cad_(iopt)) of the crankshaft within a first one ofthe subranges (L_(II)) by a strategy (II) of varying the effectivecompression ratio (c) in the cylinder within a range bounded downwardsby a lowest acceptable compression ratio c_(min)), and within a secondone of the subranges (L_(III)) by a strategy (III) which entails leadingcooled exhaust gases (ceg) to the combustion chamber in a quantity alsoenabling within the second subrange (L_(III)) control of theself-ignition of the fuel mixture towards the optimum crankshaft angle(cad_(iopt)) of the crankshaft by variation of the effective compressionratio (c) within the range bounded downwards by the lowest acceptablecompression ratio (C_(min)).
 2. An arrangement according to claim 1,wherein the control unit is adapted to regulating the effectivecompression ratio (c) in the cylinder by initiating variable closure ofthe inlet valve (ivc).
 3. An arrangement according to claim 2, furthercomprising a hydraulic control system operable for controlling thevariable inlet valve closure (ivc).
 4. An arrangement according to claim1, further comprising an exhaust line of the combustion engine from theexhaust valve an inlet line to the inlet valve for air supply to thecombustion chamber, and a return line extending from the exhaust line tothe inlet line.
 5. An arrangement according to claim 4, wherein thereturn line comprises a valve for controlling the supply of exhaustgases to the inlet line.
 6. An arrangement according to claim 4, whereinthe return line further comprises a cooler operable for cooling theexhaust gases before they reach the inlet line.
 7. An arrangementaccording to claim 1, further comprising a first sensor for detecting aparameter (p) which indicates the start of a combustion process in thecombustion chamber, and a second sensor for estimating the crankshaftangle (cad) of the crankshaft, and the control unit is operable fordetermining the crankshaft angle (cad_(i)) for the start of thecombustion process.
 8. An arrangement according to claim 7, wherein thefirst sensor is a pressure sensor operable for detecting the pressure(p) in the combustion chamber.
 9. An arrangement according to claim 7,wherein the control unit is operable for comparing the estimatedcrankshaft angle (cad_(i)) at the self-ignition of the combustionprocess with stored information concerning the optimum crankshaft angle(cad_(iopt)) for self-ignition of the combustion process and for usingthat information for controlling the self-ignition of the followingcombustion process.
 10. An arrangement according to claim 1, furthercomprising an injection nozzle operable for injecting fuel into thecombustion chamber when the inlet valve is open.
 11. A method forcontrolling a combustion engine wherein the combustion engine comprisesa combustion chamber, a movable piston in the combustion chamber and thepiston being movable to compress a fuel mixture in the combustionchamber so that self-ignition of the fuel mixture takes place, acrankshaft connected to and driven by movements of the piston; an inletvalve to the combustion chamber and an exhaust valve from the combustionchamber; the method comprising: controlling the self-ignition of thefuel mixture towards an optimum crankshaft angle (cad_(iopt)) of thecrankshaft within a load range (L_(tot)), that is divided into at leastfirst and second subranges (L_(II), L_(III)); controlling theself-ignition of the fuel mixture towards the optimum crankshaft angle(cad_(iopt)) within the first subrange (L_(II)) by a strategy (II) ofvarying the effective compression ratio (c) in the cylinder within arange bounded downwards by a lowest acceptable compression ratio(C_(min)), and within the second subrange (L_(III)) by a strategy (III)of leading cooled exhaust gases (ceg) to the combustion chamber in aquantity also enabling within the second subrange (L_(III)) control ofthe self-ignition of the fuel mixture towards the optimum crankshaftangle (cad_(iopt)) of the crankshaft by variation of the effectivecompression ratio (c) within the range bounded downwards by the lowestacceptable compression ratio (C_(min)).
 12. A method according to claim11, further comprising regulating the effective compression ratio in thecylinder by initiating variable inlet valve closure (ivc).
 13. A methodaccording to claim 12, further comprising controlling the variable inletvalve closure (ivc) by a hydraulic control system.
 14. A methodaccording to claim 11, further comprising leading cooled exhaust gases(ceg) to the combustion chamber via a return line extending from anexhaust line of the combustion engine from the exhaust valve to an inletline to the inlet valve for air supply to the combustion chamber.
 15. Amethod according to claim 14, further comprising valve controlling thesupply of exhaust gases to the inlet line.
 16. A method according toclaim 14, further comprising cooling the exhaust gases before they reachthe inlet line.
 17. A method according to claim 11, further comprisingdetermining the crankshaft angle (cad_(i)) at the start of thecombustion process by detecting a parameter (p) which is related to thecombustion process in the combustion chamber, and by detecting thecrankshaft angle (cad) of the combustion engine.
 18. A method accordingto claim 17, wherein the parameter detected is the pressure (p) in thecombustion chamber.
 19. A method according to claim 17, furthercomprising comparing the estimated crankshaft angle (cad_(i)) at thestart of the combustion process with stored information concerning theoptimum crankshaft angle (cad_(iopt)) for the start of the combustionprocess, and using that information for controlling the self-ignition ofthe following combustion process.
 20. A method according to claim 11,further comprising injecting fuel into the combustion chamber when theinlet valve is open.